1. Field of the Invention
The present invention relates to a gas sensor control apparatus which includes a gas sensor element employing electrodes formed of an electrically conductive oxide.
2. Description of the Related Art
Conventionally, a gas sensor has been known which includes a gas sensor element whose electrical characteristic changes with the concentration of a particular gas component contained in a gas under measurement. For example, Patent Document 1 discloses a gas sensor element which includes a solid electrolyte member having the shape of a bottomed tube closed at its forward end; an inside electrode (a reference electrode) formed on the inner surface of the solid electrolyte member; and an outside electrode (a detection electrode) formed on a forward end portion of the outer surface of the solid electrolyte member. Such a gas sensor is suitably used for detecting the concentration of a particular gas contained in exhaust gas discharged from, for example, a combustor or an internal combustion engine.
Importantly, the output of a gas sensor greatly depends on the temperature of its gas sensor element (hereinafter referred to as the “element temperature”). Therefore, a technique of maintaining the element temperature within a predetermined temperature range (allowable range) has been used and within which the gas sensor element is in an active state (Patent Document 2). For example, a technique which utilizes a phenomenon whereby the element impedance (internal resistance) of the gas sensor element changes with the element temperature has been used. According to this technique, a heater for heating the gas sensor element is provided, and the supply of electric current to the heater is feedback-controlled such that the element impedance (internal resistance) becomes equal to a target impedance (target resistance) (hereinafter also referred to as an “internal resistance control”). Notably, in conventional gas sensor elements, from the viewpoint of, for example, durability and catalytic performance, a noble metal such as platinum (Pt) is often used as an electrode material.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2009-63330
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2013-231659
3. Problem to be Solved by the Invention
However, it has been known that when internal resistance control is performed using a gas sensor element in which a noble metal such as platinum (Pt) serves as an electrode material, a change in the resistance of the electrode leads (hereinafter referred to as “lead resistance”) with temperature may adversely affect the stability of the element temperature. In general, the internal resistance of a gas sensor element is represented by the sum of the lead resistance and a detection portion resistance (grain internal resistance+grain boundary resistance+electrode interface resistance). For example, the resistance of platinum has a positive correlation with temperature, and the detection portion resistance has a negative correlation with temperature. In the case where the entire internal resistance has a negative correlation with temperature, the element temperature can be maintained within the predetermined range by controlling the internal resistance to a fixed value. However, since the resistance of platinum has a positive correlation with temperature, when the amount of change in the lead resistance with temperature is large, a problem arises in that even when internal resistance control is performed, the control accuracy of the element temperature is not sufficient. Consequently, the element temperature cannot be controlled to a predetermined range. Particularly, in the case where the amount of a noble metal such as platinum used as an electrode material is reduced for cost reduction whereby the lead resistance increases, the above-mentioned problem becomes remarkable.